Effect of Cobalt on Precipitation in Fe-Cr-Co-Mo-Ni-C Stainless Steels

Abstract Nano-scale precipitation is critical for martensitic steels to achieve ultra-high strengths. To date, there is a lack of in-depth understanding about tailoring Co addition to optimize strengthening precipitations in Fe-Cr-Co-Mo-Ni-C steels. This research reveals that a high addition of Co in a Fe-Cr-Co-Mo-Ni-C steel significantly promotes multiple precipitation of Mo-rich intermetallic compounds (IMCs), carbides and α’-Cr, and achieve an exceptional hardening effect during aging at 540 ℃. In particular, a variation of Co content from 5.1 to 12.8 wt.% enhances α’-Cr precipitation, and accelerates precipitation kinetics of Mo-rich phase during early-stage aging. However, few influences were observed on the solubility of Mo and equilibrium fraction of Mo-rich phases, according to both long-term aged microstructures and thermodynamic equilibrium calculation.